Optical communication system, optical receiver, optical receiver control method, and non-transitory computer readable medium

ABSTRACT

Present invention provides an optical communication system that controls reception sensitivity of an optical receiver. The communication system ( 100 ) according to the present invention comprising: an optical transmitter ( 1 ) to which an transmission signal is input, and which modulates the transmission signal to an optical signal and transmits the optical signal; and an optical receiver ( 2 ) that receives the optical signal and demodulates the optical signal to an transmission signal. And the optical receiver ( 2 ) includes a photoelectric conversion means ( 10 ) for converting the optical signal into an analog electric signal, a conversion and demodulation means ( 25 ) for converting the analog electric signal into a digital signal and demodulating the signal to the transmission signal, and an amplitude control means ( 102 ) for controlling amplitude of the analog electric signal, and the amplitude control means ( 102 ) controls the amplitude of the analog electric signal in accordance with wavelength dispersion of the optical signal.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This application is a continuation of U.S. application Ser. No.14/904,023, filed on Jan. 8, 2016, which is a National Stage Entry ofInternational Application No. PCT/JP2014/002737, filed May 26, 2014,which claims priority from Japanese Patent Application No. 2013-145488,filed Jul. 11, 2013. The entire contents of the above-referencedapplications are expressly incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to an optical communication system, anoptical receiver, an optical receiver control method, and anon-transitory computer readable medium.

BACKGROUND ART

Along with an increase in demand of data communication service in recentyears, the introduction of a longer-distance, larger-volume high-densitywavelength multiplexing optical fiber communication system with higherreliability is being promoted. On such a background, higher performanceof an optical communication system that uses optical fibers is beingdemanded. As part thereof, the adoption of a digital coherent receptionsystem in which a digital processing technique is introduced to anoptical receiver is being promoted.

In the digital coherent reception system, it is possible to correct, bydigital processing, linear degradation of an optical waveform due tooptical fiber transmission, and compensate transmission characteristicdegradation due to a wavelength dispersion characteristic of an opticalfiber as a transmission channel. However, due to the wavelengthdispersion, an optical signal waveform collapses and expands in anamplitude direction, so in a digital conversion, a dynamic range for ananalog/digital conversion becomes a problem.

As a communication system that takes the wavelength dispersioncharacteristic into consideration, Patent Literature 1 discloses amethod of changing a dynamic range at a time when an analog electricsignal is converted into a digital electric signal by monitoringwavelength dispersion.

CITATION LIST

Patent Literature 1: Japanese Unexamined Patent Application PublicationNo. 2012-129960

SUMMARY OF INVENTION Technical Problem

In the method of changing the dynamic range, when the wavelengthdispersion is small, control is performed to reduce the dynamic range.Accordingly, a resolution in the analog/digital conversion is notchanged, and sensitivity at a time when the digital electric signal isdemodulated is not also changed.

The purpose of the present invention is to provide an opticalcommunication system that controls reception sensitivity of an opticalreceiver in accordance with wavelength dispersion.

Solution to Problem

An example of a communication system comprising:

an optical transmitter to which an transmission signal is input, andwhich modulates the transmission signal to an optical signal andtransmits the optical signal; and

an optical receiver that receives the optical signal and demodulates theoptical signal to an transmission signal, wherein

the optical receiver includes a photoelectric conversion means forconverting the optical signal into an analog electric signal, aconversion and demodulation means for converting the analog electricsignal into a digital signal and demodulating the signal to thetransmission signal, and an amplitude control means for controllingamplitude of the analog electric signal, and

the amplitude control means controls the amplitude of the analogelectric signal in accordance with wavelength dispersion of the opticalsignal.

An example of an optical receiver comprising:

a photoelectric conversion means for converting an optical signal intoan analog electric signal;

a conversion and demodulation means for converting the analog electricsignal into a digital signal and demodulating the signal to antransmission signal; and

an amplitude control means for controlling amplitude of the analogelectric signal,

wherein the amplitude control means controls the amplitude of the analogelectric signal in accordance with wavelength dispersion of the opticalsignal.

An example of an optical receiver control method,

wherein an optical receiver is configured to convert an optical signalinto an analog electric signal, convert the analog electric signal intoa digital signal, and demodulate the signal to an transmission signal,

the control method comprising controlling amplitude of the analogelectric signal in accordance with wavelength dispersion of the opticalsignal.

An example of a non-transitory computer readable medium for causing acomputer to execute an optical receiver control method, wherein

an optical receiver is configured to convert an optical signal into ananalog electric signal, convert the analog electric signal into adigital signal, and demodulate the digital signal to an transmissionsignal, and

the control method includes controlling amplitude of the analog electricsignal in accordance with wavelength dispersion of the optical signal.

Advantageous Effects of Invention

According to the aspects described above, it is possible to provide anoptical communication system that controls the reception sensitivity ofthe optical receiver.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a structural example of an optical communication system 100according to first embodiment of the present invention.

FIG. 2 is a diagram showing the structure of an optical transmitter 1according to first embodiment of the present invention.

FIG. 3 is a diagram of an optical receiver 2 according to firstembodiment of the present invention.

FIG. 4 is a diagram of an amplitude control unit 102 according to firstembodiment of the present invention.

FIG. 5 is a flowchart of a method for communication according to firstembodiment of the present invention.

FIG. 6 is a sequence diagram of a method for communication according tofirst embodiment of the present invention.

FIG. 7 is a diagram of a communication system 300 according to secondembodiment of the present invention.

FIG. 8 is a diagram of an optical transmitter 3 according to secondembodiment of the present invention.

FIG. 9 is a diagram of an optical receiver 4 according to secondembodiment of the present invention.

FIG. 10 is a diagram of a amplitude control unit 302 according to secondembodiment of the present invention.

FIG. 11 is a flowchart of a communication method according to secondembodiment of the present invention.

FIG. 12 shows a communication system as a technique to which the presentinvention is relevant.

DESCRIPTION OF EMBODIMENTS

First, studied matters about relevant techniques until the presentinvention has been conceived will be described.

FIG. 12 shows a communication system 500 as a technique to which thepresent invention is relevant. A digital coherent receiver 560 shown inFIG. 12 receives optical signal transmitted from an optical transmitter550, causes local light to be interfered in a 90-degree hybrid circuit522, converts the light into an analog electric signal by coherentdetection in PDs (Photo Diodes) 523, sets signal amplitude to beconstant in analog amplifiers 524, and inputs the signal to an ADC(Analog/Digital Converter) unit 501 of a DSP (Digital Signal Processor)525.

The analog amplifier 524 is subjected to gain control in such a mannerthat average amplitude of the analog electric signal that is convertedto the electric signal is constant, but the waveform of the opticalsignal received is expanded in a temporal axis direction by beingaffected by wavelength dispersion of an optical fiber as a transmissionchannel. The wavelength dispersion is such a phenomenon that, duringtransmission of optical pulses in an optical fiber, light beams withdifferent wavelengths are transmitted at different speeds in the opticalfiber, and thus a width of an optical signal is temporally expanded.Then, a number of waveforms that are expanded in the temporal axisdirection are superposed, thereby increasing the amplitude of areception signal and causing the waveform to change at random.

To perform digital processing for an analog signal in the DSP 525, inthe ADC unit 501, it is necessary to perform a digital signal conversionfor entire analog signal amplitude. Accordingly, it is necessary tocontrol an output of the analog amplifier 524 in such a manner that apeak (maximum value) of the analog signal amplitude input to the ADCunit 501 is equal to or less than a dynamic range (width between theminimum value and the maximum value of an analog signal input rangewhich can be identified) of the ADC unit 501.

As a method of controlling the gain (input/output ratio) of the analogamplifier 524, generally, used is such a method that the outputamplitude of the analog amplifier 524 is monitored, a comparator 527compares the output amplitude and a preset reference value output from areference generation circuit 526 with each other, and a feedback isgiven to the gain control for the analog amplifier 524.

In the relevant technique, the reference value output from the referencegeneration circuit 526 is fixed, and the reference value is set inaccordance with the maximum value of wavelength dispersion of thetransmission channel. Therefore, even in the case where a dispersionvalue is small, and distortion of a signal waveform is small, the signalamplitude is controlled to be small. In the case where the wavedispersion is small, and the distortion of the signal waveform is small,it is possible to increase the signal amplitude and enhance theresolution in the ADC relatively. However, because the signal amplitudeis set to be constant, the transmission signal amplitude to the ADC unit501 is not changed, which is a problem in that the reception sensitivityof the optical receiver is not changed as compared to the case where thewavelength dispersion is at maximum.

In a relevant digital coherent reception circuit, the input amplitude ofthe ADC is uniformly set irrespective of a wavelength dispersion value.In the case where the wavelength dispersion value of the transmissionchannel is large, the waveform of a reception signal is distorted due tothe wavelength dispersion, resulting in an increase in the peak of theamplitude. Further, performing uniform amplitude control for thereception signal results in falling out of the dynamic range of the ADC.For this reason, when the reference value that is input to thecomparator 527 is set to be small, and the analog signal amplitude thatis input to the ADC is set to be small, it is confirmed that thereceptions sensitivity is improved.

However, in the case where the reference value is set to be small, thesignal amplitude after dispersion compensation also becomes small. Inthe case where the wavelength dispersion of the transmission channel issmall, the distortion of the waveform is also small, and the peak of theamplitude is also small. But, there is a problem in that the analogsignal amplitude input to the ADC remains small, and the receptionsensitivity is not improved. The present invention has been made on thebackground as described above, and embodiments of the present inventionwill be described below.

Embodiment 1

FIG. 1 shows a structural example of an optical communication system 100according to this embodiment. FIG. 1 shows an optical transmitter with aDP-QPSK (Dual Polarization-Quadrature Phase Shift Keying) system and anoptical receiver with a digital coherent system. The opticalcommunication system 100 is provided with a DP-QPSK transmitter (opticaltransmitter) 1 and a digital coherent DP-QPSK receiver (opticalreceiver) 2.

FIG. 2 is a diagram showing the structure of the optical transmitter 1.The optical transmitter 1 is provided with a circuit for generating andtransmitting a DP-QPSK signal. The optical transmitter 1 is providedwith an LD (Laser Diode) 11 that generates CW (Continuous Wave) light asa light source of signal light and a DP-QPSK modulator 12 that performsDP-QPSK modulation for the CW light from the LD 11 with an inputelectric signal (input signal).

The DP-QPSK modulator 12 divides the CW light into two light beams,performs QPSK modulation for each light beam, then orthogonalizespolarization planes at 90 degrees, and performs multiplexing, therebygenerating a polarization multiplexed signal. The optical transmitter 1indicates the structure of a general DP-QPSK transmitter.

FIG. 3 is a diagram showing the structure of the optical receiver 2. Theoptical receiver 2 is provided with a photoelectric conversion unit 101to which an optical signal is input, and which causes the signal to beinterfered with local light and converts the optical signal into anelectric signal, a conversion and demodulation unit (DSP: Digital SignalProcessor) 25 that converts the analog electric signal input from thephotoelectric conversion unit 101 into a digital signal and performswavelength dispersion compensation and demodulation of the DP-QPSKsignal by digital processing, and an amplitude control unit 102.

The photoelectric conversion unit 101 is provided with a local lightgeneration unit (LO; Local Oscillator) 21 that generates local light, a90-degree hybrid circuit 22 for inputting and causing the receivedDP-QPSK signal and the local light from the LO 21 to interfere with eachother, PDs (Photo Diodes) 23 that perform coherent detection for theoptical signal interfered in the 90-degree hybrid circuit 22 and convertthe signal into an analog electric signal, and analog amplifiers (TIAs;Trans Impedance AMPs) 24 that input the analog electric signal from thePDs 23 and amplify the analog electric signal to a predeterminedamplitude by a gain control signal from a comparator (COMP; Comparator)27.

FIG. 3 shows a block diagram of the internal structure of the conversionand demodulation unit 25. The conversion and demodulation unit 25 isprovided with an ADC unit 201 that converts the analog electric signalinto a digital signal, a dispersion estimation and compensation unit 202that estimates and compensates wavelength dispersion and polarizationdispersion, a carrier estimation unit 203 that extracts a carrier fromthe signal that has been digital conversion and reproduces a clock, anda QPSK demodulation unit 204 that demodulates a QPSK signal andgenerates an electric signal (output signal) having the same waveform asan original signal.

Here, the polarization dispersion is such a phenomenon that, due todistortion of an optical fiber, a reflection direction of light in thefiber is divided into slow components and rapid components, with theresult that a difference is generated in the time of arrival, and asignal component width is increased.

The ADC unit 201 monitors average amplitude of the input analog electricsignal and outputs a monitored value to the comparator 27.

The dispersion estimation and compensation unit 202 outputs a dispersionestimation result to a reference generation circuit 26. The carrierestimation unit 203 detects phase states of the signal light and locallight of the signal that has been subjected to the dispersioncompensation and performs phase correction. The QPSK demodulation unit204 demodulates the phase-corrected signal and restores the signal to adata signal that is identical to the transmission signal in the opticaltransmitter.

FIG. 4 is a block diagram showing the internal structure of theamplitude control unit 102. The amplitude control unit 102 is providedwith the reference generation circuit 26 to which a dispersionestimation result is input from the conversion and demodulation unit 25and which generates a reference value and the comparator (COMP) 27 towhich an amplitude monitored value of the analog electric signal isinput from the conversion and demodulation unit 25, and which comparesthe value with the reference value input from the reference generationcircuit 26 and outputs a gain control signal to the analog amplifier 24.

As shown in FIG. 4, the reference generation circuit 26 is provided withan input unit 104 serving as an interface to which the dispersionestimation value output from the dispersion estimation and compensationunit 202 is input, a computation unit 105 that calculates an optimalreference value on the basis of the dispersion estimation value, and anoutput unit 106 serving as an interface for outputting the referencevalue to the comparator 27. The structure described above is merely anexample, and another device structure may be used.

As shown in FIG. 4, the comparator 27 is provided with an input unit 107serving as an interface to which the reference value output from thereference generation circuit 26 and the amplitude monitored value outputfrom the ADC unit are input, a comparison unit 108 that compares thereference value and the amplitude monitored value with each other, andan output unit 109 serving as an interface for outputting a gain controlsignal based on a result of the comparison between the reference valueand amplitude monitored value to the analog amplifier 24. The structuredescribed above is merely an example, and another device structure maybe used.

Explanation of Processing

Subsequently, with reference to FIG. 2 and FIG. 3, processing of acommunication system 100 according to this embodiment will be described.The optical transmitter 1 uses an input electric signal to performDP-QPSK modulation for CW light from the LD 11 and transmits the lightto the transmission channel. The optical receiver 2 inputs a DP-QPSKoptical signal received from the transmission channel and local lightgenerated in the local light generation unit 21 to the 90-degree hybridcircuit 22. The 90-degree hybrid circuit 22 respectively divides theinput optical signal and local signal into four and adjusts thepolarization and the phase in such a manner that the DP-QPSK signal canbe demodulated to multiplex the optical signal and the local light.

The 90-degree hybrid circuit 22 inputs the multiplexed light to the fourPDs 23 and causes the signal light and the local light to interfere witheach other, thereby converting phase modulation into amplitudemodulation and generating an analog electric signal. The analog electricsignal is faint, so the signal is amplified to a predetermined amplitudeby the analog amplifiers 24. At this time, the comparator 27 uses acontrol signal to adjust a gain of the analog amplifier 24 in such amanner that the signal amplitude in the input of the ADC unit 201 in theconversion and demodulation unit 25 becomes constant.

FIG. 5 is a flowchart showing the processing of a method for controllingthe communication system 100. The optical signal transmitted from theoptical transmitter 1 is received by the optical receiver 2. The opticalreceiver performs computation of a wavelength dispersion estimationvalue of the received optical signal (S100). The optical receiver 2compares the wavelength dispersion estimation value with thepredetermined reference value to determine whether the wavelengthdispersion estimation value is larger than the reference value or not(S101).

In the case where the optical receiver 2 determines that the wavelengthdispersion value is larger than the reference value (S101: yes), theoptical receiver 2 sets the reference value to be small (S102). On thebasis of the reference value, the optical receiver 2 controls theamplitude of the analog electric signal to be small (S103). In the casewhere the optical receiver 2 determines that the dispersion value issmaller than the reference value (S101: no), the optical receiver 2 setsthe reference value to be large (S104). On the basis of the referencevalue, the optical receiver 2 controls the amplitude of the analogelectric signal to be large (S105).

FIG. 6 is a sequence diagram showing processing of each unit of theoptical receiver 2.

As an initial state, the reference generation circuit 26 generates areference value in the case where the wavelength dispersion estimationvalue is at the maximum, that is, a minimum reference value, and thecomparator 27 uses the reference value to set the gain of the analogamplifier 24. Here, in the case of the wavelength dispersion of theinput transmission signal is at the maximum, the maximum peak value ofthe signal amplitude is obtained. The comparator 27 performs control toset the gain of the TIA 24 to be small with the minimum reference value,so the input amplitude to the ADC unit 201 does not exceed the dynamicrange.

The analog amplifier 24 amplifies the analog electric signal and inputsthe signal to the conversion and demodulation unit 25.

The conversion and demodulation unit 25 samples the input analogelectric signal in the ADC unit 201 and converts amplitude informationinto a digital signal.

The ADC unit 201 monitors the amplitude of the analog electric signaland outputs a monitored value to the comparator 27. The ADC unit 201performs sampling on the basis of the waveform of the analog signal,converts the analog signal into a digital signal, and inputs the signalto the dispersion estimation and compensation unit 202. The dispersionestimation and compensation unit 202 estimates a wavelength dispersionvalue and a polarization dispersion value by a numerical valuecalculation and compensates the wavelength dispersion on the basis ofthe estimation result.

The dispersion estimation and compensation unit 202 also outputs thewavelength dispersion estimation value to the reference generationcircuit 26. The carrier estimation unit 203 detects phase states of thesignal light and the local light of the signal that has been subjectedto the dispersion compensation and performs phase correction. The QPSKdemodulation unit 204 demodulates the signal that has been subjected tothe phase correction and restores the signal to the original signal.

The reference generation circuit 26 inputs the wavelength dispersionestimation value from the dispersion estimation and compensation unit202 of the conversion and demodulation unit 25 through the input unit104. The input unit 104 outputs the wavelength dispersion estimationvalue to the computation unit 105. The computation unit calculates anoptimal reference value corresponding to the wavelength dispersion valuein such a manner that the input amplitude to the ADC unit 201 isoptimized in accordance with the wavelength dispersion estimation valuefrom the dispersion estimation and compensation unit 202. As thereference value computation means, a table may be referred to, orcalculation may be used. The computation unit outputs the referencevalue to the comparator 27 through the output unit 106.

Here, in the case where the wavelength dispersion of the transmissionsignal is smaller than a predetermined reference value, a differencebetween signal amplitude after the dispersion compensation and a peakvalue of signal amplitude of an electric signal before the dispersioncompensation becomes small. As a result, in order to enhance receptionsensitivity of a signal, it is possible to increase the input amplitudeof the analog signal to the ADC unit 201. In view of this, in signalprocessing in the conversion and demodulation unit 25, the gain of theTIA is increased so as not to be outside of the dynamic range, and theinput amplitude to the ADC unit is optimized.

The comparator 27 inputs the amplitude monitored value of the analogelectric signal from the ADC unit 201 of the conversion and demodulationunit 25 and the reference value from the reference generation circuit 26through the input unit 107. The input unit 107 outputs the amplitudemonitored value and the reference value to the comparison unit 108. Thecomparison unit 108 compares the amplitude monitored value and thereference value with each other.

Through the output unit 109, the comparison unit 108 performs control toset the gain of the analog amplifier 24 to be small in the case wherethe amplitude monitored value is larger than the reference value andperforms control to set the gain of the analog amplifier 24 to be largein the case where the amplitude monitored value is smaller than thereference value.

That is, in the case where the amplitude monitored value is larger thanthe reference value, the comparison unit 108 performs control to set theamplitude of the analog electric signal to be small. Further, in thecase where the amplitude monitored value is smaller than the referencevalue, the comparison unit 108 performs control to set the amplitude ofthe analog electric signal to be large. Note that in the case where thereference value is equal to the amplitude monitored value, thecomparison unit 108 does not control the amplitude of the analogelectric signal. By this control, the analog amplifier 24 makes anadjustment in such a manner that the amplitude of the analog signal tobe output is equal to the reference value output from the referencegeneration circuit 26.

In the analog electric signal output from the analog amplifier 24, awaveform thereof is distorted due to the dispersion of the transmissionchannel, a temporal distribution of the signal is expanded along with anincrease in the wavelength dispersion value, and a variation of theamplitude is increased due to superposition of the signals. As a result,in the case where the wavelength dispersion of the transmission channelis large, the peak value of the electric signal before the dispersioncompensation is increased relative to the signal amplitude after thedispersion compensation. On the other hand, in the case where thewavelength dispersion value of the transmission channel is small, thetemporal distribution of the signal is small, and the variation of theamplitude becomes small, so it is possible to increase the inputamplitude of the analog signal to the ADC unit 201 within an acceptablerange of the ADC unit 201.

Further, as an example, the control method for the amplitude of theanalog electric signal may be a method as follows: in the case where afirst amplitude corresponding to a first wavelength dispersion value asa reference is provided, if a second wavelength dispersion value issmaller than the first wavelength dispersion value, control is performedto set a second amplitude to be larger relative to the first amplitude,and if the second wavelength dispersion value is larger than the firstwavelength dispersion value, control is performed to set the secondamplitude to be smaller relative to the first amplitude.

Explanation of Effects

According to this embodiment, the wavelength dispersion value of thereception signal estimated by the dispersion estimation and compensationunit 202 of the conversion and demodulation unit 25 is input to thereference generation circuit, and in accordance with the estimatedwavelength dispersion value, the reference value can be changed. As aresult, the gain control for the analog amplifier 24 is performed insuch a manner that the input amplitude of the analog electric signal tothe ADC unit 201 in accordance with the wavelength dispersion value ofthe reception signal, making it possible to optimally adjust thereception sensitivity.

Through the processing described above, the DP-QPSK optical receiver 2adjusts the amplitude of the analog electric signal in the input of theADC unit 201 of the optical receiver 2 to be the optimal value at alltimes in accordance with the wavelength dispersion value of thetransmission channel of the optical signal, with the result that areduction in the reception sensitivity of the signal can be avoided.

Embodiment 2

In Embodiment 1, the example by the DP-QPSK communication system isdescribed. In this embodiment, an example using a DP-BPSK (DualPolarization-Binary Phase Shift Keying) system is described. FIG. 7shows a structural example of an optical communication system 300according to this embodiment. FIG. 7 shows a structure in the case wherethe embodiment of the present invention is applied to a DP-BPSKmodulation system. In FIG. 7, an optical transmitter and a digitalcoherent receiver with the DP-BPSK system are shown as an example. Theoptical communication system 300 is provided with a DP-BPSK transmitter(optical transmitter) 3 and a digital coherent DP-BPSK receiver (opticalreceiver) 4.

FIG. 8 is a diagram showing the structure of the optical transmitter 3.The optical transmitter 3 is provided with a circuit for generating andtransmitting a DP-BPSK signal. The optical transmitter 3 is providedwith a LD (Laser Diode) 31 that generates CW (Continuous Wave) light asa light source of signal light and a DP-BPSK modulator 32 that performsDP-BPSK modulation for CW light of the LD 31 with an input electricsignal (input signal).

In the DP-BPSK modulator 32, the CW light is divided into two lightbeams, the two light beams are respectively subjected to BPSKmodulation, then polarization planes are orthogonalized at 90 degrees,and performs multiplexing, thereby generating a polarization multiplexedsignal. The optical transmitter 3 indicates the structure of a generalDP-BPSK transmitter.

FIG. 9 is a diagram showing the structure of the optical receiver 4. Theoptical receiver 4 is provided with a photoelectric conversion unit 301to which an optical signal is input and which causes the light to beinterfered with local light and converts the optical signal into anelectric signal, a conversion and demodulation unit 45 that converts theanalog electric signal that is input from the photoelectric conversionunit 301 into a digital signal and performs wavelength dispersioncompensation and demodulation for the DP-BPSK signal by digitalprocessing, and an amplitude control unit 302.

The photoelectric conversion unit 301 is provided with a local lightgeneration unit (LO; Local Oscillator) 41 that generates local light, a90-degree hybrid circuit 42 to which the local light from the LO 41 andthe received DP-BPSK signal are input and which causes the light and thesignal to be interfered with each other, PDs (Photo Diodes) 43 thatperform coherent detection for the optical signal interfered in the90-degree hybrid circuit 42 and convert the signal into an analogelectric signal, and analog amplifiers (TIAs; Trans Impedance AMPs) 44to which the analog electric signal from the PDs 43 is input and whichamplify the analog electric signal to a predetermined amplitude by again control signal from a comparator (COMP; Comparator) 47.

FIG. 9 shows a block diagram of the internal structure of the conversionand demodulation unit 45. The conversion and demodulation unit 45 isprovided with an ADC unit 401 that converts the analog electric signalinto a digital signal, a dispersion estimation and compensation unit 402that performs estimation and compensation for wavelength dispersion andpolarization dispersion, a carrier estimation unit 403 that extracts acarrier from the signal that has been subjected to the digitalconversion and reproduces a clock, and a BPSK demodulation unit 404 thatdemodulates a BPSK signal to generate an electric signal (output signal)having the same waveform as the original signal.

The ADC unit 401 monitors average amplitude of the input analog electricsignals and outputs a monitored value to the comparator 47.

The dispersion estimation and compensation unit 402 outputs a dispersionestimation result to a reference generation circuit 46. The carrierestimation unit 403 detects phase states of the local light and thesignal light that have been subjected to the dispersion compensation andperforms phase correction. The BPSK demodulation unit 404 demodulatesthe phase-corrected signal and restores the signal to an transmissionsignal having the same waveform as the original signal.

FIG. 10 is a diagram showing the internal structure of the amplitudecontrol unit 302. The amplitude control unit 302 is provided with thereference generation circuit 46 to which a wavelength dispersionestimation result is input from the conversion and demodulation unit 45and which generates a reference value and the comparator (COMP) 47 towhich the amplitude monitored value of the analog electric signal isinput from the conversion and demodulation unit 45 and which comparesthe reference value that is input from the reference generation circuit46 therewith and outputs a gain control signal to the analog amplifier44.

As shown in FIG. 10, the reference generation circuit 46 is providedwith an input unit 304 serving as an interface to which the wavelengthdispersion estimation value output from the dispersion estimation andcompensation unit 402 is input, a computation unit 305 that calculatesan optimal reference value on the basis of the wavelength dispersionestimation value, and an output unit 306 serving as an interface foroutputting the reference value to the comparator 47. The structuredescribed above is merely an example, and another device structure maybe used.

As shown in FIG. 10, the comparator 47 is provided with an input unit307 serving as an interface to which the reference value output from thereference generation circuit 46 and the amplitude monitored value outputfrom the ADC unit are input, a comparison unit 308 that compares thereference value and the amplitude monitored value with each other, andan output unit 309 serving as an interface for outputting a gain controlsignal based on a result of the comparison between the reference valueand the amplitude monitored value to the analog amplifier 44. Thestructure described above is merely an example, and another devicestructure may be used.

Subsequently, with reference to FIG. 11 and FIG. 6, processing of thecommunication system 300 according to this embodiment will be described.The optical transmitter 3 uses the input electric signal to perform theDP-BPSK modulation for the CW light from the LD 31 and transmits thelight to the transmission channel. The optical receiver 4 inputs theDP-BPSK optical signal input from the transmission channel and the locallight generated in the local light generation unit 41 to the 90-degreehybrid circuit 42. The 90-degree hybrid circuit 42 divides the inputoptical signal and the local light into two, respectively, adjusts thepolarization and the phase so as to be able to demodulate the DP-BPSKsignal, and multiplexes the optical signal and the local light.

The 90-degree hybrid circuit 42 inputs the multiplexed light to the twoPDs 43 and causes the signal light and the local light to be interferedwith each other, thereby converting the phase modulation into anamplitude modulation and generating an analog electric signal. Theanalog electric signal is faint, so the signal is amplified to apredetermined amplitude by the analog amplifiers 44. At this time, thecomparator 47 uses a control signal to adjust a gain of the analogamplifier 44 in such a manner that the signal amplitude in the input ofthe ADC unit 401 in the conversion and demodulation unit 45 becomesconstant.

FIG. 11 is a flowchart showing the processing of a communication methodfor the communication system 300. The optical signal transmitted fromthe optical transmitter 3 is received by the optical receiver 4. Theoptical receiver performs computation of a wavelength dispersionestimation value of the received optical signal (S200). The opticalreceiver 4 compares the wavelength dispersion estimation value with thepredetermined reference value to determine whether the wavelengthdispersion estimation value is larger than the reference value or not(S201).

In the case where the optical receiver 4 determines that the dispersionvalue is larger than the reference value (S201: yes), the opticalreceiver 4 sets the reference value to be small (S202). On the basis ofthe reference value, the optical receiver 4 controls the amplitude ofthe analog electric signal to be small (S203). In the case where theoptical receiver 4 determines that the dispersion value is smaller thanthe reference value (S201: no), the optical receiver 4 sets thereference value to be large (S204). On the basis of the reference value,the optical receiver 4 controls the amplitude of the analog electricsignal to be large (S205).

The processing of the units of the optical receiver 4 is the same as thesequence diagram shown in FIG. 6. Hereinafter, as shown in FIG. 6, theprocessing of each component of the communication system 300 will bedescribed in detail.

As an initial state, the reference generation circuit 46 generates areference value in the case where the wavelength dispersion estimationvalue is at the maximum, that is, a minimum reference value, and thecomparator 47 uses the reference value to set the gain of the analogamplifier 44. In the case of the maximum wavelength dispersion of thetransmission signal, the maximum peak value of the signal amplitude isobtained. The comparator 47 performs control to set the gain of the TIA44 to be small with the minimum reference value, so the input amplitudeto the ADC unit 401 does not exceed the dynamic range.

The analog amplifier 44 amplifies the analog electric signal and inputsthe signal to the conversion and demodulation unit 45. The conversionand demodulation unit 45 samples the input analog electric signal in theADC unit 401 and converts amplitude information to a digital signal.

The ADC unit 401 monitors the amplitude of the electric signal andoutputs a monitored value to the comparator 47. The ADC unit 401performs sampling on the basis of the waveform of the analog signal,converts the analog signal into a digital signal, and inputs the signalto the dispersion estimation and compensation unit 402. The dispersionestimation and compensation unit 402 estimates a wavelength dispersionvalue and a polarization dispersion value by a numerical valuecalculation and compensates the dispersions on the basis of theestimation result.

The dispersion estimation and compensation unit 402 also outputs thewavelength dispersion estimation value to the reference generationcircuit 46. The carrier estimation unit 403 detects phase states of thesignal light and the local light of the signal that has been subjectedto the dispersion compensation and performs phase correction. The BPSKdemodulation unit 404 demodulates the signal that has been subjected tothe phase correction and restores the signal to the original signal.

The reference generation circuit 46 inputs the wavelength dispersionestimation value from the dispersion estimation and compensation unit402 of the conversion and demodulation unit 45 through the input unit304. The input unit 304 outputs the wavelength dispersion estimationvalue to the computation unit 305. The computation unit calculates anoptimal reference value corresponding to the wavelength dispersion valuein such a manner that the input amplitude to the ADC unit 301 becomesoptimal, in accordance with the wavelength dispersion estimation valuefrom the dispersion estimation and compensation unit 402. As thereference value computation means, a table may be referred to, orcalculation may be used. The computation unit outputs the referencevalue to the comparator 47 through the output unit 306.

Here, in the case where the wavelength dispersion of the transmissionsignal is smaller than the predetermined reference value, a differencebetween signal amplitude after the dispersion compensation and a peakvalue of signal amplitude of an electric signal before the dispersionbecomes small. As a result, in order to enhance reception sensitivity ofa signal, it is possible to increase the input amplitude of the analogsignal to the ADC unit 301. In view of this, in signal processing in theconversion and demodulation unit 45, the gain of the TIA is increased soas not to be outside of the dynamic range, and the input amplitude tothe ADC unit is optimized.

The comparator 47 inputs the amplitude monitored value of the analogelectric signal from the ADC unit 301 of the conversion and demodulationunit 45 and the reference value from the reference generation circuit 46through the input unit 307. The input unit 307 outputs the amplitudemonitored value and the reference value to the comparison unit 308. Thecomparison unit 308 compares the amplitude monitored value and thereference value with each other.

Through the output unit 309, the comparison unit 308 performs control toset the gain of the analog amplifier 44 to be small in the case wherethe amplitude monitored value is larger than the reference value andperforms control to set the gain of the analog amplifier 44 to be largein the case where the amplitude monitored value is smaller than thereference value.

That is, in the case where the amplitude monitored value is larger thanthe reference value, the comparison unit 308 performs control to set theamplitude of the analog electric signal to be small. Further, in thecase where the amplitude monitored value is smaller than the referencevalue, the comparison unit 308 performs control to set the amplitude ofthe analog electric signal to be large. Note that in the case where thereference value is equal to the amplitude monitored value, thecomparison unit 308 does not control the amplitude of the analogelectric signal. By this control, the analog amplifier 44 makes anadjustment in such a manner that the amplitude of the analog signal tobe output is equal to the reference value output from the referencegeneration circuit 46.

In the analog electric signal output from the analog amplifier 44, awaveform thereof is distorted due to the dispersion of the transmissionchannel, a temporal distribution of the signal is expanded along with anincrease in the wavelength dispersion value, and a variation of theamplitude is increased due to superposition of the signals. As a result,in the case where the wavelength dispersion of the transmission channelis large, the peak value of the electric signal before the dispersioncompensation is increased relative to the signal amplitude after thedispersion compensation. On the other hand, in the case where thewavelength dispersion value of the transmission channel is small, thetemporal distribution of the signal is small, and the variation of theamplitude becomes small, so it is possible to increase the inputamplitude of the analog signal to the ADC unit 401 within an acceptablerange of the ADC unit 401.

Further, as an example, the control method for the amplitude of theanalog electric signal may be a method as follows: in the case where afirst amplitude corresponding to a first wavelength dispersion value asa reference is provided, if a second wavelength dispersion value issmaller than the first wavelength dispersion value, control is performedto set a second amplitude to be larger relative to the first amplitude,and if the second wavelength dispersion value is larger than the firstwavelength dispersion value, control is performed to set the secondamplitude to be smaller relative to the first amplitude.

Explanation of Effects

According to this embodiment, the wavelength dispersion value of thereception signal which is estimated by the dispersion estimation andcompensation unit 402 of the conversion and demodulation unit 45 isinput to the reference generation circuit, and in accordance with theestimated wavelength dispersion value, the reference value can bechanged. As a result, it is possible to perform the gain control for theanalog amplifier 44 in such a manner that the input amplitude to the ADCunit 401 becomes optimal in accordance with the wavelength dispersionvalue of the reception signal, and optimally adjust the receptionsensitivity.

Through the above processing, in accordance with the wavelengthdispersion value of the transmission channel of the optical signal, theDP-BPSK optical receiver 2 can adjust the amplitude of the analogelectric signal in the input of the ADC unit 401 of the optical receiver4 to an optimal value at all times, with the result that a reduction inthe reception sensitivity of the signal can be avoided.

Other Embodiments

The optical communication method described in the above embodiment isrealized using semiconductor processing units each including anapplication specific integrated circuit (ASIC). These types ofprocessing may be performed by causing a computer system including atleast one processor (e.g. microprocessor, MPU, or digital signalprocessor (DSP)) to execute a program. Specifically, these types ofprocessing may be performed by generating one or more programs includinginstructions for causing a computer system to execute an algorithm aboutthe types of processing and then providing these programs to a computer.

These programs may be stored in various types of non-transitorycomputer-readable media and then provided to a computer. Suchnon-transitory computer-readable medium include various types oftangible storage media.

Examples of the non-transitory computer-readable media include magneticstorage media (e.g. flexible disks, magnetic tapes, hard disk drives),magneto-optical storage media (e.g. magneto-optical disks), compact discread-only memory (CD-ROM), CD-R, CD-R/W, semiconductor memory (e.g. maskROM, programmable ROM (PROM), erasable PROM (EPROM), flash ROM, andrandom access memory (RAM).

The programs may be provided to a computer by various types oftransitory computer-readable media. Examples of such transitorycomputer-readable media include electric signals, optical signals, andelectromagnetic waves. Such transitory computer-readable media canprovide the programs for a computer through a wire communication pathsuch as an electric line or an optical fiber, or a wirelesscommunication path.

Note that the present invention is not limited to the above embodimentsand can be appropriately changed without departing from the gist of thepresent invention. For example, the present invention may be applied toa quadrature amplitude modulation (QAM), an orthogonal frequencydivision multiplexing (OFDM), or other digital communication systems.

Further, the above embodiment is only illustrative of the application ofthe technical idea obtained by the present inventors. That is, thetechnical idea, is not limited to only the above embodiment, and variouschanges can, of course, be made to the embodiment.

For example, part or all of the above embodiment can be described as theSupplementary Notes below, but the embodiment is not limited thereto.

(Supplementary Note 1)

A communication system which comprises an optical transmitter to whichan transmission signal is input, and which modulates the transmissionsignal to an optical signal and transmits the optical signal: and

an optical receiver that receives the optical signal and demodulates theoptical signal to an transmission signal, wherein

the optical receiver includes a photoelectric conversion means forconverting the optical signal into an analog electric signal, aconversion and demodulation means for converting the analog electricsignal into a digital signal and demodulating the signal to thetransmission signal, and an amplitude control means for controllingamplitude of the analog electric signal, and

the amplitude control means controls the amplitude of the analogelectric signal in accordance with wavelength dispersion of the opticalsignal.

(Supplementary Note 2)

The communication system according to Supplementary Note 1, wherein theamplitude control means controls the amplitude of the analog electricsignal to become smaller, as the wavelength dispersion of the opticalsignal is increased.

(Supplementary Note 3)

The communication system according to Supplementary Note 1 or 2, wherein

the photoelectric conversion means includes an amplifier that amplifiesthe analog electric signal input to the conversion and demodulationmeans, and

the amplitude control means estimates a wavelength dispersion value ofthe optical signal and determines, on the basis of the estimatedwavelength dispersion value, a reference value for controlling a gain ofthe amplifier.

(Supplementary Note 4)

An optical receiver comprising:

a photoelectric conversion means for converting an optical signal intoan analog electric signal;

a conversion and demodulation means for converting the analog electricsignal into a digital signal and demodulating the signal to antransmission signal; and

an amplitude control means for controlling amplitude of the analogelectric signal,

wherein the amplitude control means controls the amplitude of the analogelectric signal in accordance with wavelength dispersion of the opticalsignal.

(Supplementary Note 5)

The optical receiver according to Supplementary Note 4, wherein theamplitude control means controls the amplitude of the analog electricsignal to become smaller, as the wavelength dispersion of the opticalsignal is increased.

(Supplementary Note 6)

The optical receiver according to Supplementary Note 4 or 5, wherein

the photoelectric conversion means includes an amplifier that amplifiesthe analog electric signal input to the conversion and demodulationmeans, and

the amplitude control means estimates a wavelength dispersion value ofthe optical signal and determines, on the basis of the estimatedwavelength dispersion value, a reference value for controlling a gain ofthe amplifier.

(Supplementary Note 7)

An optical receiver control method,

wherein an optical receiver is configured to convert an optical signalinto an analog electric signal, convert the analog electric signal intoa digital signal, and demodulate the signal to an transmission signal,

the control method comprising controlling amplitude of the analogelectric signal in accordance with wavelength dispersion of the opticalsignal.

(Supplementary Note 8)

The optical receiver control method according to Supplementary Not 7,wherein the controlling includes controlling the amplitude of the analogelectric signal to become smaller, as the wavelength dispersion of theoptical signal is increased.

(Supplementary Note 9)

The optical receiver control method according to Supplementary Note 7 or8, wherein

the optical receiver includes an amplifier that amplifies the analogelectric signal input to a conversion and demodulation means, and

the controlling includes estimating a wavelength dispersion value of theoptical signal and determining, on the basis of the estimated wavelengthdispersion value, a reference value for controlling a gain of theamplifier.

(Supplementary Note 10)

A non-transitory computer readable medium for causing a computer toexecute an optical receiver control method, wherein

an optical receiver is configured to convert an optical signal into ananalog electric signal, convert the analog electric signal into adigital signal, and demodulate the digital signal to an transmissionsignal, and

the control method includes controlling amplitude of the analog electricsignal in accordance with wavelength dispersion of the optical signal.

(Supplementary Note 11)

The non-transitory computer readable medium according to SupplementaryNot 10, wherein the controlling includes controlling the amplitude ofthe analog electric signal to become smaller, as the wavelengthdispersion of the optical signal is increased.

(Supplementary Note 12)

The non-transitory computer readable medium according to SupplementaryNote 10 or 11, wherein

the optical receiver includes an amplifier that amplifies the analogelectric signal input to a conversion and demodulation means, and

the controlling includes estimating a wavelength dispersion value of theoptical signal and determining, on the basis of the estimated wavelengthdispersion value, a reference value for controlling a gain of theamplifier.

While the invention of the present application has been described withreference to the embodiment, the invention is not limited thereto.Various changes understandable by those skilled in the art can be madeto the configuration or details of the invention of the presentapplication without departing from the scope of the invention.

The present application claims priority based on Japanese PatentApplication No. 2013-145488, filed on Jul. 11, 2013, the disclosure ofwhich is incorporated herein in its entirety.

REFERENCE SIGNS LIST

-   1 optical transmitter-   2 optical receiver-   3 optical transmitter-   4 optical receiver-   11 LD (Laser Diode)-   12 DP-QPSK modulator-   21 local light generation unit (LO; Local Oscillator)-   22 90-degree hybrid circuit-   23 PD (Photo Diode)-   24 analog amplifier (TIA; Trans Impedance AMP)-   25 conversion modulation unit (DSP)-   26 reference generation circuit-   27 comparator (COMP)-   31 LD (Laser Diode)-   32 DP-BPSK modulator-   41 local light generation unit (LO; Local Oscillator)-   42 90-degree hybrid circuit-   43 PD (Photo Diode)-   44 analog amplifier (TIA; Trans Impedance AMP)-   45 conversion and demodulation unit (DSP)-   46 reference generation circuit-   47 comparator (COMP)-   100 optical communication system-   101 photoelectric conversion unit-   102 amplitude control unit-   104 input unit-   105 computation unit-   106 output unit-   107 input unit-   108 comparison unit-   109 output unit-   201 ADC unit-   202 dispersion estimation and compensation unit-   203 carrier estimation unit-   204 QPSK demodulation unit-   300 optical communication system-   301 photoelectric conversion unit-   302 amplitude control unit-   304 input unit-   305 computation unit-   306 output unit-   307 input unit-   308 comparison unit-   309 output unit-   401 ADC unit-   402 dispersion estimation and compensation unit-   403 carrier estimation unit-   404 BPSK demodulation unit-   500 optical communication system-   501 ADC unit-   521 local light generation unit (LO; Local Oscillator)-   522 90-degree hybrid circuit-   523 PD (Photo Diode)-   524 analog amplifier (TIA; Trans Impedance AMP)-   525 conversion and demodulation unit (DSP)-   526 reference generation circuit-   527 comparator (COMP)-   550 optical transmitter-   560 optical receiver

The invention claimed is:
 1. A communication system comprising: anoptical transmitter to which a transmission signal is input, and whichmodulates the transmission signal to an optical signal and transmits theoptical signal; and an optical module that receives the optical signaland demodulates the optical signal to the transmission signal, whereinthe optical module includes a photoelectric converter configured toconvert the optical signal into an analog electric signal, an analogdigital converter configured to convert the analog electric signal intoa digital signal, a digital signal processor configured to process thedigital signal, and a controller configured to receive an amplitudevalue of the analog electric signal and a wavelength dispersion value ofthe optical signal, and wherein the controller is further configured tocontrol an amplitude of the analog electric signal based on theamplitude value and a reference amplitude value based on the wavelengthdispersion value.
 2. The communication system according to claim 1,wherein the controller is further configured to control the amplitude ofthe analog electric signal to become smaller as the wavelengthdispersion value is increased.
 3. The communication system according toclaim 1, wherein the photoelectric converter includes an amplifier thatis configured to amplify the analog electric signal input to the analogdigital converter, and the controller is further configured to estimatethe wavelength dispersion value and determine, based on the estimatedwavelength dispersion value, the reference amplitude value forcontrolling a gain of the amplifier.
 4. An optical module comprising: aphotoelectric converter configured to convert an optical signal into ananalog electric signal; an analog digital converter configured toconvert the analog electric signal into a digital signal; a digitalsignal processor configured to process the digital signal; and acontroller configured to receive an amplitude value of the analogelectric signal and a wavelength dispersion value of the optical signal,wherein the controller is further configured to control an amplitude ofthe analog electric signal based on the amplitude value and a referenceamplitude value based on the wavelength dispersion value.
 5. The opticalreceiver according to claim 4, wherein the controller is furtherconfigured to control the amplitude of the analog electric signal tobecome smaller as the wavelength dispersion value of the optical signalis increased.
 6. The optical receiver according to claim 4, wherein thephotoelectric converter includes an amplifier that is configured toamplify the analog electric signal input to the analog digitalconverter, and the controller is further configured to estimate thewavelength dispersion value and determine, based on the estimatedwavelength dispersion value, the reference amplitude value forcontrolling a gain of the amplifier.
 7. An optical module controlmethod, wherein the optical module receiver performs operationscomprising: converting an optical signal into an analog electric signal;converting the analog electric signal into a digital signal; processingthe digital signal; receiving an amplitude value of the analogelectronic signal and a wavelength dispersion value of the opticalsignal; demodulating the optical signal to a transmission signal; andcontrolling an amplitude of the analog electric signal based on theamplitude value and a reference amplitude value based on the wavelengthdispersion value.
 8. The optical module control method according toclaim 7, wherein the controlling includes controlling the amplitude ofthe analog electric signal to become smaller as the wavelengthdispersion value is increased.
 9. The optical module control methodaccording to claim 7, wherein the optical module includes an amplifierthat is configured to amplify the analog electric signal input to theanalog digital converter, and the controlling includes estimating thewavelength dispersion value and determining, based on the estimatedwavelength dispersion value, the reference amplitude value forcontrolling a gain of the amplifier.